Synchronous rectification for low voltage motor drive

ABSTRACT

An inductive load driven by power MOSFETs, such as in a low voltage motor drive, using synchronous rectification to reduce the voltage drop across a body diode of a power MOSFET. A comparative feed back circuit measures voltage across the power MOSFET to determine when the body diode is conducting, and turns the MOSFET ON during conductive cycles, and OFF to block reverse current. The obtained synchronous rectification function is highly sensitive to current flow, while using a very small number of parts in a configuration that has less complexity.

[0001] This application is based on and claims benefit of U.S.Provisional Application No. 60/327,792, filed Oct. 10, 2001, entitledSYNCHRONOUS RECTIFICATION FOR LOW VOLTAGE MOTOR DRIVE, to which a claimof priority is hereby made.

BACKGROUND OF THE INVENTION

[0002] Many power electronic applications have inductive loads that aredriven by power MOSFETs. These MOSFETs are typically in a configurationshown in FIG. 1. When one of the MOSFETs (e.g., s1) is turned off, theinductive load causes the load current to flow through the internaldiode of the second MOSFET (i.e., s2). These diodes, known as bodydiodes, have a minimum forward drop of about 0.6V. The MOSFET channel,however, due to its resistive characteristic, has a drop that isproportional to the current. Thus, in many cases turning the channel ONcauses a lower drop across the device, which in turn, reduces the powerdissipation.

[0003] This method of using the channel to carry the reverse currentflow is well known and is called synchronous rectification inapplications that create a DC output voltage such as DC-DC convertersand synchronous recirculation in applications that drive a motor. Anabstract synchronous rectifier is illustrated in FIG. 2. A switch S3 isopen to permit diode D1 to block reverse current, and closes when diodeD1 conducts a forward current. Accordingly, closure of switch S3prevents the occurrence of a forward voltage drop of about 0.7 voltsacross diode D1, and therefore avoids the attendant heat generated bycurrent flowing through diode D1.

[0004] In most cases, a controller 16 for switch S3 has to know when thecurrent reverses, so that diode D1 is forward conducting, to controlswitch S3 to turn on to eliminate the forward voltage drop. The circuitalso has to know when the switch current has diminished to zero and thenturn switch S3 OFF. If switch S3 is not turned OFF when the current goesto zero, the current may then reverse in many applications (such asSwitched Reluctance motors and trapezoidally wound and driven BrushlessDC motors) and this can be harmful to the application. In manyapplications, due to cost or technical reasons, it is difficult for thecontroller to know if the current has gone to zero to turn the switch S3OFF.

SUMMARY OF THE INVENTION

[0005] According to the present invention, there is provided a circuitfor measuring a voltage drop across a MOSFET to determine the directionof current flowing through the MOSFET. The voltage measurements acrossthe MOSFET are introduced to a comparator that also has a specialvoltage input to produce a comparative result with the MOSFET voltage.An output of the comparator is coupled to the driver for the MOSFET toact in conjunction with the basic MOSFET driver command signals. Thecomparator output provides a signal that can indicate when the MOSFETshould be conducting to turn ON the MOSFET channel and lower the forwardvoltage drop for current flowing through the channel. The comparatoroutput signal also indicates when current through the MOSFET reduces toapproximately zero and signals the MOSFET driver to turn OFF the MOSFETchannel to permit the body diode of the MOSFET to block reverse currentthrough the MOSFET.

[0006] By measuring the voltage across the MOSFET, and comparing theMOSFET voltage to a reference voltage, a signal is produced thatprovides an enhanced sensitivity for controlling the operation of aMOSFET as a synchronous rectifier. The circuit according to the presentinvention uses very few components and obtains a direct measurementacross the MOSFET to enhance the operation of the circuit as asynchronous rectifier. According to an embodiment of the presentinvention, the input voltage across the MOSFET can be filtered beforebeing supplied to the comparator to further enhance the sensitivity andnoise immunity of the circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The present invention is described in detail below with referenceto the accompanying drawings, in which;

[0008]FIG. 1 is a simplified circuit diagram of an inductive load drivenby power MOSFETs;

[0009]FIG. 2 is a circuit model of a synchronous rectifier; and

[0010]FIG. 3 is a circuit diagram illustrating synchronous rectificationaccording to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0011] The present invention is shown in FIG. 3, where the MOSFET driver2 has the required intelligence to be able to autonomously turn theMOSFET 4 ON or OFF to take advantage of synchronous recirculation.

[0012] An OR circuit 6, a comparator 8, a reference voltage 10 andfilter 12 are used with the driver to achieve the synchronousrectification function. It should be apparent that filter 12 variesdepending upon the application, and may be eliminated altogether.

[0013] The function of additional elements is to turn MOSFET 4 ON whenthe voltage across it is below a preset reference voltage (in FIG. 2,reference voltage 10) which could be a low positive (about 0.5 v) ornegative. This method takes advantage of the fact that the voltageacross MOSFET 4 is low and in fact negative when the body diode 14 ofMOSFET 4 is conducting. When comparator 8 senses that the voltage acrossMOSFET 4 has become negative, it concludes that diode 14 is ON and thenturns ON the MOSFET channel. The conducting MOSFET channel represents asignificant reduction in the forward voltage drop of the switch, andcorrespondingly reduced temperature.

[0014] When the current flowing from source to drain reduces and becomeszero, or starts to reverse, i.e., starts to flow from drain to source,the voltage across MOSFET 4 starts to increase. During this process, thebody diode 14 begins to block the reverse current. As the voltage sensedby comparator 8 crosses the set limit, it turns OFF MOSFET 4.

[0015] Comparator 8 should be able to sense negative voltages. Inaddition, comparator 8 can be configured to exhibit hysteresis, or havedifferent outputs based on whether the sensed voltage is crossing thereference voltage 10 value from a lower or higher valued direction. Invarious applications, filter 12 is coupled to comparator 8 toremove/smooth out the switching noise across MOSFET 4 to obtain acleaner sensed voltage. Filter 12 may be operated with a blankinginterval, in which the sensing of voltage across MOSFET 4 is suspendedfor some arbitrary time period. The use of a blanking interval permitsfilter 12 to be more specific to practical circuit conditions, forexample by blanking out intervals of operation known to have largetransient signals.

[0016] Reference voltage 10 is selected in accordance with the noise andthe delays required in the applications. It should be apparent thatreference voltage 10, while theoretically zero, can take on any positiveor negative value, dependent upon circuit and operation parameters. Inapplications with a high voltage on the DC bus, it may be preferable toclamp the input voltage to comparator 8 or add a divider network toreduce the input voltage.

[0017] Integrating the functionality of the present invention within thedriver has the advantage of a reduced part count and decreasedcomplexity of the controller. For example, it is possible to provide asingle half-bridge driver package incorporating the circuit of theinvention to consolidate components. In such a case, a driver packagecan include driving capability for one or two MOSFETs, whether in ahalf-bridge configuration or any other combination.

[0018] Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art. It ispreferred, therefore, that the present invention be limited not by thespecific disclosure herein.

What is claimed is:
 1. A circuit for controlling a MOSFET in asynchronous rectifier mode, comprising: a comparator circuit with aninput coupled to the MOSFET for obtaining a sensed voltage across theMOSFET; a reference voltage input to the comparator circuit forcomparison with the sensed voltage across the MOSFET; a MOSFET drivercoupled to the MOSFET for controlling channel switching of the MOSFET; acontrol signal coupled to the MOSFET driver to switch the MOSFETchannel; and an output of the comparator circuit coupled to the MOSFETdriver in conjunction with the control signal to switch the MOSFETchannel in a relationship with the sensed voltage.
 2. The circuitaccording to claim 1, further comprising a filter circuit interposedbetween the MOSFET and the comparator circuit.
 3. The circuit accordingto claim 2, further comprising a filter blanking control coupled to thefilter circuit for suspending input to the filter for a predeterminedtime period.
 4. The circuit according to claim 1, wherein the comparatorcircuit exhibits hysteresis.
 5. The circuit according to claim 1,wherein the reference voltage is zero.
 6. A low voltage motor drivecircuit with synchronous rectification, comprising: a MOSFET half bridgefor driving an inductive load coupled to a midpoint of the half bridge;a comparator circuit having an input coupled to the mid point of thehalf bridge circuit for sensing voltage across one of the MOSFETs in thehalf bridge circuit; the comparator circuit having another input coupledto a reference voltage to provide a comparative threshold for comparisonwith the sensed voltage across the MOSFET; a control circuit coupled tothe MOSFET for switching a channel of the MOSFET; and a comparatorcircuit output coupled to the control circuit for influencing switchingof the MOSFET channel, such that a responsiveness of the MOSFET operatedas a synchronous rectifier is increased.
 7. The circuit according toclaim 6, further comprising: another comparator circuit having an inputcoupled to the midpoint of the half bridge circuit for sensing voltageacross another of the MOSFETs in the half bridge circuit; anothercontrol circuit coupled to the another MOSFET for switching a channel ofthe another MOSFET; and an output of the another comparator circuitcoupled to the another control circuit for influencing switching of theanother MOSFET channel, such that a responsiveness of the half bridgeMOSFETs operated as synchronous rectifiers is increased.
 8. The circuitaccording to claim 6, further comprising a filter circuit interposedbetween the midpoint of the half bridge and the comparator for removingnoise from the sensed voltage signal across the MOSFET.
 9. The circuitaccording to claim 6, wherein the comparator circuit exhibitshysteresis.
 10. The circuit according to claim 8, further comprising afilter blanking control coupled to the filter circuit for suspendinginput to the filter for a predetermined time period.
 11. The circuitaccording to claim 6, further comprising: a driver circuit coupled tothe MOSFET for switching the MOSFET channel; a control signal input intothe driver circuit for controlling the driver to switch the MOSFETchannel; and wherein the comparator output and the control signal areORed together to control the driver circuit.
 12. A method forcontrolling a MOSFET operated as a synchronous rectifier in a lowvoltage motor drive, comprising: measuring a voltage across the MOSFETto determine current flow in the MOSFET channel; comparing the measuredvoltage across the MOSFET to a reference voltage to obtain an indicationof when the measured voltage is higher or lower than the referencevoltage; combining the indication with a control signal to operate adriver circuit for switching the MOSFET channel, such that the MOSFETchannel is turned ON when the sensed voltage across the MOSFET isgreater than the reference voltage and the MOSFET is switched OFF whenthe sensed voltage across the MOSFET is less than the reference voltage.13. A method according to claim 12, further comprising filtering thesensed voltage across the MOSFET for comparison with the referencevoltage.
 14. A method according to claim 13, further comprisingsuspending filtering for a predetermined period of time.
 15. The methodaccording to claim 12, further comprising providing a range for theindication that the sensed voltage is higher or lower than the referencevoltage, wherein the indication does not change state while thecomparison of the sensed voltage to the reference voltage is within therange thereby obtaining a hysteresis function in the comparator.
 16. Amethod for operating a MOSFET as a synchronous rectifier in a halfbridge of a low voltage motor drive, comprising: determining whencurrent is flowing in a forward direction through a body diode of theMOSFET; switching the MOSFET ON based on the determination that currentis flowing in a forward direction; determining when the body diode isblocking a reverse current through the MOSFET; and switching OFF theMOSFET channel based on the determination that the body diode of theMOSFET is blocking current.
 17. A half bridge driver for switchingMOSFETs in a half bridge, comprising; a MOSFET driver coupled to a firstMOSFET in the half bridge for switching the first MOSFET; a comparatorcircuit having an input coupled to the first MOSFET for sensing avoltage across the first MOSFET; a reference voltage input to thecomparator circuit to provide a threshold voltage level for comparisonto the sensed voltage across the first MOSFET; a control signal coupledto the MOSFET driver for controlling switching of the first MOSFET; andan output of the comparator circuit coupled to the MOSFET driver incombination with the control signal to influence switching of the MOSFETin relation to the second voltage.
 18. The half bridge driver accordingto claim 17, wherein the MOSFET driver and the comparator circuit arecombined in a single integrated package.
 19. The half bridge driveraccording to claim 18, wherein the integrated package further includesthe first MOSFET.